The present invention relates to data communications and in particular to a system for the improved measurement of impairments in a data communication channel employing quadrature amplitude modulation--QAM or phase shift keying modulation PSK.
The performance of QAM and PSK data transmission systems can be evaluated qualitatively by the subjective evaluation of received data signal space diagrams. U.S. Pat. No. 4,035,625 for example refers to a quantitative technique for measuring channel impairments by a subjective evaluation of eye diagrams and discusses the technique in some detail.
With the increased usage of QAM and PSK modulated transmission systems, which has occurred in recent times, qualitative analysis of the signal constellation diagrams (sample eye diagrams on a 2-dimensional plane) has been used as a means of evaluating the performance of the various types of QAM and PSK systems.
In FIGS. 1 and 2 herein there are shown respectively 8 and 4 point idealized signal constellations or signal space diagrams. The horizontal axis of these diagrams may be referred to as the real, "X" or in-phase channel axis. The vertical axis may be referred to as the imaginary, "Y" or quadrature channel axis. FIG. 1 depicts an 8 point signal space diagram obtained by combining the 4 point signal space diagrams depicted in FIGS. 2A and 2B. 16 point diagrams are also obtainable as discussed in the above noted U.S. Pat. No. 4,035,625.
The signal space diagrams may be typically viewed by a technician or operator on an oscilloscope screen by modulating the horizontal axis with the X-signal and the vertical axis with Y-signal.
Degradation of the communication channel manifests itself on the diagram in relatively well defined modes, the more common of which are depicted in FIGS. 3A-3D which in its ideal form would correspond to FIG. 2A. Thus, if the communication medium is degraded by gaussian noise, the small clearly defined dots of the receiver signal space diagram depicted in FIG. 2A would enlarge to roughly circular areas as a result of the noise having been added to the data signal. This is shown in FIG. 3A wherein the diameter of the circular areas is a measure of the noise. Phase jitter contributed by the communications media results in the clear defined dots of FIG. 2A being displaced as an arc centered at the ideal points. This is depicted in FIG. 3B. Frequency offset and harmonic distortion cause the unique changes in the diagram as depicted in FIGS. 3C and 3D respectively.
As noted, since the various distortions manifest themselves in a unique fashion on the signal space diagram, it would appear that a qualitative determination of the communications channel impairment could be made by study of the diagram. In actual practice, however, the situation becomes complicated by the fact that the degrading influences can be cumulative and thus the clearly defined patterns depicted in FIGS. 3A-3D become distorted by the interaction of the combination of degrading factors. In addition, the signal space diagram data does not directly lend itself to accurate quantitative analysis of the condition of the communications channel.
Copending, commonly assigned application Ser. No. 16,912 filed Mar. 2, 1979 and entitled "System for the Quantitative Measurement of Impairments in the Communication Channel of a Quadrature Amplitude Modulation Data Communication System" addresses itself to a system for evaluating data transmission over a communication media in the face of the interaction of the degrading influences. In addition to the interaction of degrading influences another type of interference is also present in these systems which is referred to as intersymbol interference. This type of interference is caused by an overlap in time between successive received data symbols; the extraneous energy from the signal in one or more keying intervals tending to interfere with the reception of the signal in another keying interval. A technique to reduce the influence of intersymbol interference can be utilized in modem designs since this channel impairment can be determined and as such can be removed through the use of automatic equalizer technology. FIG. 4A shows a block diagram of the receiver of a general modem in which a complex equalizer is utilized after the digital conversion but prior to the decision algorithm in order to reduce the influence of intersymbol interference. The invention in application Ser. No. 16,912 considered that system of FIG. 4A, specifically including the equalizer, is utilized and as such that the scatter of the space diagram due to line impairments could be quantized and analysis could be performed on the quantized data without consideration of the influence of intersymbol interference. There are however commercially available demodulator integrated circuits typified by the block diagram of FIG. 4B which do not include the implementation of automatic equalizers to remove intersymbol interference. Such interference being manifest, the analysis of signal space patterns becomes increasingly complicated.